Method of making high density iron powder

ABSTRACT

IRON POWDER OF AN APPARENT DENSITY OF OVER 3.0 GMS./ CC. IS PRODUCED BY FIRST SHOTTING MOLTEN IRON TO OBTAIN SUBSTANTIALLY SPERICAL SHOT, SCREENING THE SHOT TO REMOVE OVERSIZE PARTICLES AS WELL AS VERY FINE PARTICLES, SINTERING THE SHOT IN THE PRESENCE OF A DECARBONIZING AGENT, GRINDING THE SINTERED MATERIAL AND AGAIN SCREENING TO REMOVE PARTICLES ABOVE A CERTAIN SIZE.

United States Patent 01 3,597,188 METHOD OF MAKING HIGH DENSITY IRON POWDER Nathan Neumann, Ridgway, Pa., assiguor to Domtar Limited, Montreal, Quebec, Canada No Drawing. Filed Aug. 23, 1968, Ser. No. 754,991 Int. Cl. B22c 9/00 US. Cl. 75.5BA 4 Claims ABSTRACT OF THE DISCLOSURE Iron powder of an apparent density of over 3.0 -gms./ cc. is produced by first shotting molten iron to obtain substantially spherical shot, screening the shot to remove oversize particles as well as very fine particles, sintering the shot in the presence of a decarbonizing agent, grindr'ng the sintered material and again screening to remove particles above a certain size.

The present invention relates to the production of dense iron powder and, more particularly, to a method for producing iron powder of a high apparent density from cast iron shot.

For many powder metallurgical applications an iron powder of low apparent density, i.e. less than 2.6 gms./ cc., is required. Recently iron powders of a much higher apparent density of between about 3.3 and 3.8 gms./ cc. have been in demand particularly for the production of high-yield welding electrodes. These welding electrodes consist of a core surrounded by a coating containing iron powder, which results in welding electrodes having yields greater than 180% and up to about 250%. The term yield means the weight of the welding material deposited as a percentage of the weight of the core wire.

Much of the iron powder produced commercially is dry-reduced iron powder, i.e. iron powder produced by reducing iron oxide, sintering it to make a sponge iron cake, then crushing, grinding and screening the cake to form the powder. Normally, the apparent density is no higher than about 2.6 gms./ cc. Attempts to increase the density by cold-working and repeated milling, followed by a heat treatment in a protective atmosphere have resulted in an increase to only about 2.8 gms./cc., which is still too low to be suitable for the production of high yield electrodes. Recently, a method has been described in Canadian Pat. No. 729,556, whereby the sponge iron is subjected to an annealing treatment between coldworking and final milling, and it is reported that an iron powder of an apparent density of at least 3.3 gms./cc. is produced. While this method can be used to advantage in increasing the apparent density of sponge iron, it is not applicable to cast iron shot obtained from a shotting atom operation. 1

Conventional processes of producing iron powders by shotting include in sequence: a shotting of molten iron, grinding the shot so formed, sintering and decarbonizing the ground shot and finally grinding the sintered decarbonized product to a powder. These processes produce iron powders with apparent densities of less than about 3 gms./ cc. Prior to the present invention, no process of forming high density iron powder from iron shot was available.

It is thus the object of the present invention to provide a method of producing a dense iron powder from cast iron formed into shot.

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Applicant has found that by omitting the grinding step, directly processing the shot from the shotting operation by screening same to obtain the proper size and size distribution, sintering and decarbonizing this shot and then grinding and screening the resultant sintered product, an iron powder having an apparent density of over 3.0 gms./cc. can be produced.

It is believed that the main factors contributing to the formation of high density powder are: the shape of the shot formed, the size of the shot, the density of the shot, and the size distribution of the shot and of the sintered product.

According to the present invention, molten iron is shotted to form high density spherical shot particles, having a specific size and size distribution, by directing jets of fluid at streams of molten iron to break up and freeze the iron into small spheres. For a given shotting operation the spherical particles so formed vary in size considerably but the particle size distribution seems to remain substantially constant. When water jets are used to freeze the iron and form the spheres, the size range of the of the particles formed changes when the jet pressure of the nozzle is changed. Generally the higher the pressure the smaller the spherical particles formed. Thus when the shotting operation is changed to materially affect the particle size and size distribution, the size of particles selected for sintering and decarbonization may require slight adjustments to produce the higher density powders. Generally shot having no particles greater than about a U5. standard sieve No. 12 and having a minimum of about 50% of the particles greater than sieve No. is suitable for the present invention. The material is screened through a No. 30 sieve to remove large particles. The very fine material must also be removed since otherwise the fine content will be too high and prevent proper packing thereby lowering the apparent density of the powder. Applicant has found that generally if the fine material passing through a No. 200 sieve, preferably a No. sieve, is separated and the retained material processed, a powder having the required density will be formed.

After screening to remove the oversize and fines, the shot is blended with a decarbonizing agent (preferably mill scale) and is sintered at a temperature in the range of about l8002200 F. for 13 hours to form a sintered product of the desired carbon content. The sintered and decarbonized material is next crushed and ground to form a powder and the powder is subjected to a second screening step to remove the larger particles. Generally the powder passing through a No. 40 sieve has been found to have a satisfactory density in the range of 3.2- 3.4 gms./cc. To further increase the density of the powder to 3.5-3.7 gms./cc. the powder may be further reduced in size in a blender.

IIt is believed that the higher apparent density of applicants product may be attributed to the spherical shape of the particles and to the particle size distribution obtained when the shot is subjected to screening operations removing the oversize and fine particles.

The following example of one specific shotting and screening operation serves to facilitate understanding of the present invention but is not intended to be limiting, since under different shotting arrangements the screen size may vary and may require slight changes in the size of material selected to be processed.

Three streams of molten scrap metal formed through 7 orifices were shot using a pair of opposed water dis- 3 charge nozzles operating at about 300 psi. The two nozzles were arranged so that their downwardly directed streams each intersected the metal streams at an angle of 29.5 to the verticle to form a V-shape. The resulting shot had the following average particle size distribution as determined by screen analysis of samples.

Sieve No. Percent retained (US. Standard): of total product +12 Nil +100 60 +200 28 200 12 Next the shot was screened through a No. 30 sieve to remove oversize and to leave shot having a size distribution as indicated by the following average screen analysis:

Sieve No. Percent retained (US. Standard): of total product 2 +80 35 +100 10 +140 14 -200 30 200 9 Next, the shot which passed through the No. 30 sieve was passed over a No. 170 sieve and the fines were removed. The material that did not pass through the No. 170 sieve had the following average screen analysis:

Sieve No. Percent retained (US. Standard): of total product 2 +80 48 +100 14 ;+140 21 +200 10 +200 5 The above shot that did not pass through the No. 170 sieve screen was blended with ground mill scale in proportions of 28 /2 parts of mill scale to 100 parts of shot to obtain a mixture having the following apparent density and particle size distribution:

Apparent density (Hal1)3.6 to 3.7 gms./cc.

Sieve No. Percent (US. Standard): of total product +40 6 +80 44 +100 10 +200 13 200 27 Apparent density (Hall)-3.35 to 3.65 gms./cc. Flow (Hall)24 seconds Colour-bluish-grey 4 Sieve No. Percent (US. Standard): of total product +30 Nil +40 .5 +50 220 38 0 10.0

Chemical analysis: Percent by weight C max .08 H loss max 1.20 S max .025 'P max .025 Mn max .40

Si max .12 V max .05 Fe (total) min 98 A higher apparent density may be obtained by subjecting the powder to a blending action, for example in a double cone blender. A sample of the powder obtained in accordance with the foregoing example was transferred to a double cone blender and after a few rotations of the blender (1 to 10 revolutions) the density increased by about .05 to .1 gms./cc. to a density in the area of 3.45 to 3.5 gms./cc. It has been found that even higher densities can be obtained by extending the blending time.

I claim:

1. A method for producing iron powder of high apparent density, which comprises; atomizing a stream of molten iron by impingement with a jet of water to produce substantially spherical shot having a particle size less than about No. 12 sieve, with at least about 50% of the shot particles retained on No. 80 sieve, removing from said shot particles greater than No. 30 sieve and particles smaller than No. 200 sieve, sintering said shot in the presence of a decarburizing agent at a temperature between about 1800 and 2200 F. for l to 3 hours thereby to produce a decarburized sintered iron powder, grinding the said powder and removing particles greater than No. 40 sieve from said powder, thereby to produce an iron powder having an apparent density of at least 3.0 gms./ cc.

2. A method according to claim 1 wherein said decarbonizing agent is mill scale.

3. A method according to claim 1 wherein said shot is sintered in the presence of mill scale in the proportion of 20 to 25% mill scale to 80 to 75% screened shot.

4. A method as defined in claim 3 wherein said iron powder has a density of 3.2-3.7 gms./cc.

References Cited UNITED STATES PATENTS 2,784,073 3/ 1957 Michalke 750.5 3,073,695 1/ 1963 Silbereisen et al 750.5 3,212,876 10/ 1965 Hulthn et al 750.5 3,325,277 6/ 1967 Huseby 750.5 3,368,890 2/ 1968 Schroeder et a1 750.5 3,385,694 5/1968 Josefsson et al 750.5 3,418,104 12/1968 Hatcher 75--0.5 3,418,105 12/1968 Hatcher et al. 75-0.5

L. DEWAYNE RUTLEDGE, Primary Examiner W. W. STALLARD, Assistant Examiner US. Cl. X.R. 750.5C 

